I'm having the same problem, also tested with multiple (working) SIMs. It was working on 'buntu 12.04 but not since upgrading to 'buntu 14.04. I have lived with using my phone as an AP when out and about, but now I really need to get this working! I had hoped that a clean upgrade to 16.04 would resolve the issue, but alas the issue remains exactly the same; the CDC is detected and the driver is loaded during boot, but shortly thereafter the device is unregistered:

*bump*ing this in the hope someone can help. And no, installing Modem Manager GUI won't make the slightest bit of difference, since the modem is "unregistered" by the kernel during boot. The question is why this happens...